1. Technical Field
The present invention relates to an interferometric distance measurement device.
2. Background Information
An example of such a device is known from German patent disclosure DE 10 2007 016 774 A1, for example. This device includes a light source, and a scanning unit with a scanning plate in the form of a transparent glass plate. A splitter, which splits a beam emitted by the light source into at least one measurement beam and at least one reference beam, are disposed on the glass plate. A reflector spaced apart in the direction of the propagation direction of the beams is also provided. The resultant interference signals are detected via a detector arrangement. The interference signals are due to the superposition of the measurement beam and the reference beam. The measurement beam is propagated in the direction of the reflector and from there is reflected back to the scanning plate. The reference beam is guided solely within the scanning plate and finally, at a unification site with the measurement beam, it is made to undergo interferential superposition. From the interference signals obtained from this, the distance between the scanning plate and the reflector, and the changes in the distance between these components, can be ascertained in a known manner. Such devices can be used, for instance, in semiconductor production equipment, for example so that, besides the two-dimensional displacement information obtained with regard to a wafer table, additional information regarding possible tilting of the table can also be made available. What is disadvantageous in the device known from DE 10 2007 016 774 A1 is especially that in the event of tilting of the reflector and scanning unit, the result is erroneous distance signals, and the degree of modulation and signal amplitudes of these signals can be a major disruption if even only slight tilting occurs.
In FIG. 9a, the fringe pattern that results in the untilted state in the detection plane of a device according to the prior art is shown in schematic form. It is for scanning this fringe pattern that the detector arrangement is typically optimized.
FIGS. 9b and 9c, also in schematic form, now show how this fringe pattern in the detection plane of a device analogous to the prior art varies, if tilting of the reflector and scanning unit results as mentioned above. As FIG. 9b shows, upon slight tilting (1 mrad) about a first axis, the fringe pattern periodicity varies. As FIG. 9c shows, in the event of once again only slight tilting (1 mrad) about a second axis, the orientation of the fringe pattern in the detection plane varies as well. In both cases, the result is the aforementioned erroneous, poor-quality distance signals.